Currently, in a manufacturing process of semiconductor device, a photolithography technique is generally used to form a pattern on a semiconductor substrate. In the photolithography technique, a predetermined pattern can be formed on photosensitive resin film by using a photomask pattern through a demagnifying projection aligner. Herein, `photomask` means an original mask for exposure on which a pattern of transparent region and masking region is formed, and, though it is called `reticle` especially except when a demagnification ratio is 1:1, hereinafter, these are commonly referred to as `photomask`.
Conventional photolithography techniques are directed to the development of an aligner, in particular, a projection lens with a high N.A. (numerical aperture) and exposure light with a shorter wavelength .lambda. to form a finer pattern on a semiconductor device. Here, N.A. corresponds to how much divergent light a lens can collect. In other words, the greater N.A. of the lens is, the more the divergent light can be collected, i.e., the more the lens performance can be improved. Moreover, the greater N.A. of the lens is or the shorter wavelength .lambda. the exposure light has, the higher the threshold resolution of photosensitive resin film can be improved, thereby providing a finer pattern mask which is used as an etching mask for the fabrication of a semiconductor device.
As described above, by realizing such high N.A. of projection lens and a short wavelength of exposure light, a high resolution can be provided and therefore a fine pattern may be theoretically obtained. However, due to the high N.A. and short wavelength, a focal depth, which means a range that allows a variation of focal position, has been decreased. Therefore, slight movement of focal position may cause a failure in forming a predetermined pattern on photosensitive resin film. Namely, the decreased focal depth makes the formation of a finer pattern difficult.